Most European countries (including Greece, Italy, and Spain) are part of the Schengen Area, a travel zone that allows movement between member countries without border checks. U.S. citizens can enter the Schengen Area visa-free for up to 90 days within any 180-day period.
It’s a group of 29 European countries (as of 2025) that have abolished internal borders for travel. Once you enter one Schengen country, you can move freely between others without additional immigration checks.